Material scientists have been exploring the properties of fullerenes which are geometric structures built of carbon atoms. In 1991, a new fullerene joined the buckyball, a cage-like structure built of 60 carbon atoms. Scientists found that the buckyball structure can be extended to form long slender tubes—carbon nanotubes—single molecules comprised of rolled graphene (graphite-like) sheets capped at each end. Thus, carbon nanotubes, the newest fullerene structure, are effectively buckyballs played out as long strands, so thin they can not be seen under an ordinary microscope and certainly not with the naked eye. In fact, it is suggested by Boris I. Yakobson and Richard E. Smalley in American Scientist (July–August 1997), “Fullerene Nanotubes: C1,000,000 and Beyond,” that when Roger Bacon used the electric arc in the early 1960s to make “thick” carbon whiskers, the nanotube discovery was a matter of looking more closely at the smallest products hidden in the soot, but Bacon lacked the high-power microscope required to see them.
To comprehend the size of a single-wall carbon nanotube, imagine holding in your hand a wand with a hollow core that is a single molecule. Such a wand would be a few atoms in circumference. In fact, nanotubes sufficient to span the 250,000 miles between the earth to the moon could be loosely rolled into a ball the size of a poppyseed. Together, the smallness of the nanotubes and the chemical properties of carbon atoms packed along their walls in a honeycomb pattern are responsible for their fascinating and useful qualities and, present significant production challenges.
Precursors to the development of carbon nanotubes are reported in patents issued prior to 1991. U.S. Pat. No. 4,025,689 discloses a process requiring the use of inert gas and temperatures up to 3000° C. for preparing a light-weight, hollow carbon body having outstanding properties as activated carbon. U.S. Pat. No. 4,228,142 prepares a high density carbon coated silicon carbide particle which can be substituted for commercial-grade diamonds by reacting a fluorocarbon and silicon carbide in an inert gas atmosphere at tempertures>800° C. U.S. Pat. No. 4,855,091 discloses the formation of a fishbone-like graphite layer along an axis of carbon filaments when a carbon containing gas is heated to temperatures between 250° C. and 800° C. in the presence of ferromagnetic metal particles. U.S. Pat. No. 5,165,909 discloses the growth of hollow carbon fibrils having layers and a core made up of concentric rings, like a tree. Metal-containing particles are used as catalysts in reactions with inexpensive, readily available carbon containing raw materials; high temperature graphitizing reactions are avoided. The fibrils have a high surface area, high tensile strength and modulus required in reinforcement applications. U.S. Pat. No. 5,747,161 discloses a method for forming a tubular shaped carbon filament less than 30 nanometers in diameter using an arc discharge process.
In summary, the pursuit of carbon structures is well documented and the foundation is laid for the use of processes employing inert gas atmospheres, metal catalysts having an affinity for carbon such as iron, nickel, cobalt (Fe, Ni, Co, respectively) and high temperatures to create the desired structure.
After the discovery of carbon nanotubes in 1991, scientific efforts have been devoted to the production of carbon nanotubes in higher yield; the production of carbon nanotubes with consistent dimensions, e.g., diameter and length; processes which separate nanotubes from other reaction products; processes which eliminate the entanglement of tubes with each other and the development of useful applications. For example, U.S. Pat. No. 5,346,683 discloses uncapping and thinning carbon nanotubes to provide open compartments for inserting chemicals. U.S. Pat. No. 5,456,986 discusses the production of magnetic nanoparticles and nanotubes from graphite rods packed with magnetic metal oxides or rare earths and subjected to carbon arc discharge. U.S. Pat. No. 5,543,378 discloses carbon nanostructures which encapsulate a palladium crystallite, allowing the delivery of substances suitable for x-ray diagnostic imaging in a safe, encapsulated form. Another method of producing encapsulated nanoparticles, nanotubes and other closed carbon structures is disclosed in U.S. Pat. No. 5,780,101 and its divisional counterpart, U.S. Pat. No. 5,965,267; a transition metal catalyst is contacted with a gas mixture containing carbon monoxide in a temperature range between 300° C. and 1000° C. Methods for isolating and increasing the yield of carbon nanotubes are disclosed in U.S. Pat. No. 5,560,898 which discusses a physical separation technique; U.S. Pat. No. 5,641,466 reveals a method for purification of carbon nanotubes by the oxidation of co-existing, but undesired carbon structures; and U.S. Pat. No. 5,698,175 claims a chemical technique for separating nanotubes from carbon by-products.
The references reveal that prior art methods for producing carbon nanotubes give undesirably low yields. Carbon nanotubes with significant variations in structure and size are usually produced and often include carbon materials of different shapes which may be carbon nanoparticles and amorphous carbon. Carbon nanotubes are further classified into one with a single hexagonal mesh tube called a single-walled nanotube (abbreviated as “SWNT”), and one comprising a tube of a plurality of layers of hexagonal meshes called a multiwalled nanotube (abbreviated as “MWNT”).
The type of carbon nanotube structure available is determined to some extent by the method of synthesis, catalysts and other conditions. Research continues in an effort to produce carbon nanotubes of a consistent, predictable structure.
The present invention contributes a more consistent, predictable method for manufacturing a particular configuration of carbon nanotubes. Novel process conditions and reactants are disclosed. The present invention also provides a solution to problems associated with handling and manipulating the “small” wand which is only visible with high-power electron microscopes, or other costly visual aids. Through the process of the present invention, a “graphitic outer layer” defined as carbon material comprising one or more distinct structures, is intentionally formed during carbon nanotube production and becomes an integral part of the carbon nanotube device. The carbon material can be either a soft amorphous carbon, a hard graphitic carbon, or a combination thereof. If the soft amorphous carbon is formed prior to the formation of the harder, more resilient graphitic carbon, the amorphous carbon serves as a cushion between the carbon nanotube and the harder graphitic carbon. The carbon material, either singly or collectively, is called the “graphitic outer layer” and creates bulk such that the submicroscopic nanotube can be handled easily and efficiently.
Computer simulations and laboratory experiments show that carbon nanotubes have extraordinary resilience, strength and various unusual electronic and mechanical properties; for instance, they can be formed into very strong ropes and can be used as probes because of a very large Young's modulus, even greater than diamond. They also exhibit electrical conductivity in a quantized fashion that has led to experiments with tiny nanowires and nanoscale transistors.